A motor drive system, in one known form, comprises an AC source supplying three-phase AC power to a variable frequency drive (VFD). The VFD includes an AC/DC converter connected by a DC link or bus to a DC/AC converter. The DC/AC converter may comprise a pulse width modulated inverter using insulated GATE bipolar transistors (IGBTs).
Many industrial applications use reciprocating loads that are driven by VFDs. In most of such applications, during certain portion of one complete load cycle, the load tends to accelerate the motor at speeds higher than that commanded by the VFD. This is referred to as “overhauling”. Under such a condition, the inverter is required to absorb the regenerated energy. Typically, this is achieved by providing braking resistor and transistor units know as ‘braking units’. The braking units are sized according to the duty cycle and stored regenerative energy in the load. Sometimes, multiple braking units may need to be employed in a master-slave manner to accommodate large amounts of regenerated energy. Use of braking units, while accepted in the industry, suffer from high costs, require large space to mount, and are inherently inefficient since the regenerated energy is dissipated as heat in the braking resistors.
Alternative methods that can handle overhauling load without the use of external brake resistors have been considered. Some of the methods include high slip braking, disabling or reducing torque component of current during overhauling, and increasing output frequency based on DC bus voltage.
Kume et al., U.S. Pat. No. 6,429,612, assigned to the assignee of the present application, discloses high slip braking. This patent relates to bringing a rotating load to a stop, rather than to absorb the regenerated energy during overhauling and continue operating in the motoring mode in the next part of the cycle.
Another technique that has been adopted in the industry for handling an overhauling type load reduces the torque-generating component of the output current during regeneration to zero. In other words, the motor is operated at approximately zero slip. Operation at zero slip yields zero regenerated torque and thus the motor is prevented from converting the mechanical energy into electrical energy. This has been implemented in high performance inverters. However, this requires an inverter capable of operating in a closed loop vector control manner that can track the actual speed of the motor and maintain operation at zero slip throughout the regenerative part and come out of this mode once this part of the cycle is complete. Such inverters need sophisticated control algorithms operating on dedicated DSP chips.
Another alternative method currently being used in the oil industry is described in Watson, U.S. Pat. No. 6,414,455. The '455 patent discusses three modes of operating a variable speed drive to handle overhauling type loads. The three modes are “TM” mode or constant torque mode, “BSM” mode or base speed mode, and “DM” mode or dual mode. In the TM mode, the speed is allowed to change in order to maintain a certain torque reference. In the BSM mode, the torque is allowed to change within the lower and upper torque limits in order to maintain the desired set speed. In the DM mode, the controller is fed with two speed reference values (Fref) and two torque reference values (Tref). The user then has the choice to either select one of the speed reference values or one of the torque reference values. The basis of the control philosophy is to limit the torque provided by the inverter to the motor during the upstroke part of the cyclic load. Doing this reduces or eliminates regenerative torque being produced during the down-stroke, thereby increasing the overall system efficiency, and reducing the size and cost of the inverter, since the inverter does not need the braking resistor unit. Some drawbacks of this method are that there is need for a DC bus voltage sensor and a DC bus current sensor. Most inverters do not have DC current sensors at the correct position and rely on the DC current flowing into the capacitor, which flows only when the capacitor voltage has gone lower than the instantaneous value of the input AC voltage and hence does not truly mirror the instantaneous torque profile of the load. Further, DC voltage across the DC bus capacitor is a parameter that changes very slowly and cannot be relied upon to take corrective action since it would be too late to act upon it. If the torque reference is maintained at a value needed for motoring action and is not changed during regenerative action, such a control philosophy will fail to limit the magnitude of the regenerated energy and this can cause a rise in DC bus voltage and eventually put the inverter in a fault state.
The present invention is directed to solving one or more of the problems discussed above.